1. Field of the Invention
The present invention generally relates to circuits for converting the characteristics of a signal, and particularly relates to a phase shifter for changing the phase of a signal.
2. Description of the Related Art
Phase shifters for changing the phase of a signal are widely used as a component for a system such as a wired/wireless communication apparatus or measurement apparatus utilizing microwave bands or milliwave bands. In the orthogonal modulation which is widely used in the wireless communication system of today, for example, a 90-degree phase shifter is used as an indispensable component in order to generate an orthogonal signal having a 90-degree phase displacement. A variable phase shifter that is capable of changing the amount of phase shift is used for various purposes such as for the phased array operation that controls the direction of transmission by supplying signals having different phases to a plurality of antenna transmitters, for the spatial synthesis operation that synthesizes in the same phase a plurality of signals received by a plurality of antennas, or for the accuracy maintaining function that electrically adjusts phases inside measurement apparatus.
Performance required of such phase shifters conventionally includes high precision, low loss, controllability, low cost, etc. As progress toward the use of higher frequencies and broader bands has been made in wireless systems in recent years, there is also a demand for increased bandwidths. As a 90-degree phase shifter required in the orthogonal modulation/demodulation system, a polyphase filter or hybrid coupler has conventionally been used.
FIGS. 1A and 1B are drawings showing a typical configuration of a polyphase filter. A polyphase filter 10 shown in FIG. 1A includes four resistors R and four capacitors C. A series connection of one resistor R and one capacitor C forms one series circuit, and four series circuits are connected in series, with the head end and the tail end being coupled to form a loop. Input terminals I1 through I4 are coupled to the connection points between every two adjacent series circuits, and output terminals O1 through O4 are coupled to the connection points between the resistor R and the capacitor C of each series circuit.
A positive-phase signal (i.e., signal having a 0-degree phase) is supplied to the input terminals I1 and I4, for example, and a negative-phase signal (i.e., signal having a 180-degree phase) is supplied to the input terminals I2 and I3. With the input signals supplied in this manner, signals having a 0-degree phase, a 90-degree phase, a 180-degree phase, and a 270-degree phase appear at the output terminals O1 through O4, respectively. Polyphase filters 10 having the same configuration as shown in FIG. 1A may be connected in cascade with the output of a given filter connected to the input of a next filter as shown in FIG. 1B. Such configuration can generate the individual phase signals in a relatively stable manner over a broad band of frequencies.
FIG. 2 is a drawing showing a typical configuration of a hybrid coupler. A hybrid coupler 11 shown in FIG. 2 is implemented by connecting transmission lines 12, 13, 14, and 15 in series to form a loop. The transmission lines 12 and 13 have a characteristic impedance of Z0, and have a length that is equal to ¼ of signal wavelength λ. The transmission lines 14 and 15 have a characteristic impedance of 0.707×Z0 (i.e., Z0/√2), and have a length that is equal to ¼ of signal wavelength λ. With an input terminal IN positioned between the transmission line 12 and the transmission line 14, for example, two signals Q0 and Q90 having a 90-degree phase difference appear at the opposite ends of the transmission line 13.
In the case of the polyphase filter as shown in FIG. 1, three to five units need to be connected in cascade in order to obtain a 90-degree phase difference over a broad band of frequencies. This results in increased circuit size. An increase in circuit size means an increase in the time required for signal propagation inside the filter, which gives rise to a problem in that a phase deviation (i.e., absolute phase deviation responsive to frequency) within the band increases. There are also an increase in loss and cost increase. In the case of the hybrid coupler shown in FIG. 2, the loss is relatively small, but this coupler is not suitable for broadband operations because of the use of ¼ wavelength transmission lines. Phase deviation (i.e., absolute phase deviation responsive to frequency) within the band is larger than in the case of the polyphase filter.
In this manner, related-art 90-degree phase shifters have a problem of poor broadband performance, i.e., a problem of a large phase deviation within the band.
Accordingly, there is a need for a phase shifter that has a small phase deviation over a broad band of frequencies.
[Patent Document 1] Japanese Patent Application Publication No. 6-69753